The present invention relates to processes for preparing protective colloid-stabilized polymers on the basis of (meth)acrylate monomers in the form of their aqueous dispersions or polymer powders redispersible in water, to the process products thus obtainable, and to the use thereof, for example, as coating materials or adhesive-bonding materials in the construction sector.
Protective colloid-stabilized polymers are employed in particular in the form of their aqueous dispersions or polymer powders redispersible in water, in numerous applications, as for example in the building sector, as coating materials or adhesive-bonding materials. Very familiar for these applications are polymers based on vinyl esters and ethylene. Protective colloids widespread in the art to date have been, in particular, partly hydrolyzed polyvinyl alcohols. In contrast, polyvinyl alcohols as protective colloids for acrylate polymers or styrene-acrylate copolymers usually do not lead to sufficiently stable polymer dispersions. There are problems, therefore, in stabilizing acrylate dispersions solely with polyvinyl alcohols in such a way that the resulting dispersions are stable and the powders, moreover, are stable to blocking (polymer stability). Generally speaking, a manageable dispersion viscosity is obtained by additional use of chain transfer agents for lowering the molecular weight, which in turn considerably impairs the blocking stability of the powders.
A further problem is that the stabilization of dispersed or redispersed particles of the polymers subsides over the course of time as a result of partly hydrolyzed polyvinyl alcohols in cementitious construction compounds. This occurs particularly in the case of acrylate polymers and styrene-acrylate copolymers. Where protective colloid-stabilized polymers are employed to improve the properties of construction compounds, however, the formulations are required to remain stable for a certain time and must not substantially change in their processing consistency (stability of viscosity, or cement stability). Otherwise, the user is unable to produce buildings having the required performance properties, such as mechanical properties, from the construction compounds, or else the user would have to produce new construction compounds again and again within short time periods, something which in the art represents an unacceptable burden.
It has further been observed that when polymers stabilized with partly hydrolyzed polyvinyl alcohols are employed in cementitious construction compounds, the stabilization of air may subside over time, meaning that air may gradually escape from the cementitious construction compounds. This effect is also dubbed “gassing” and affects the air pore content of the cementitious construction compounds and of the buildings produced from them. The air pore content and the associated porosity, however, play a key part in the mechanical properties of concrete or mortar, such as the compressive strength, for example. Too many air pores, and the compressive strength plummets; too few air pores, or none, in the mortar or concrete, and the building material lacks sufficient freeze/thaw stability.
A number of approaches have already been presented before now to the production of protective colloid-stabilized polymers for construction applications. EP-A 1065224, for instance, advises —for the preparation of poly(meth)acrylic esters—polymerization in the presence of partly hydrolyzed polyvinyl alcohols, the monomers and initiators being divided in a specific way between the initial charge and the metered feed. EP-A 0477900 teaches the addition of fully hydrolyzed 1-alkylvinyl ester/vinyl ester copolymers after the polymerization and before the drying operation, for the production of polymer powders redispersible in water. DE-A 1026074 addresses the preparation of hydrolyzed 1-alkylvinyl ester/vinyl ester copolymers and their use as protective colloid in the context of the suspension polymerization or emulsion polymerization of ethylenically unsaturated monomers. EP-A 812863 describes the preparation of poly(meth)acrylic esters by polymerization in the presence of low molecular mass polyvinyl alcohols. Polymer powders obtainable therefrom have a tendency toward blocking. From EP-A 0723975 it is possible to find protective colloid-stabilized polymer powders based on polystyrene-acrylates containing epoxy groups. EP-A 2033991, EP-A 2075286, EP-A 2166028, EP-A 2189499, EP-A 2216348, and EP-A 1612226 teach polyvinyl alcohols containing glycol side groups as protective colloids for aqueous polymer dispersions and/or water-redispersible polymer powders.
Against this background, the object was to provide stable, protective colloid-stabilized dispersions and corresponding polymer powders redispersible in water, based on (meth)acrylic ester monomers, which when used in cementitious applications possess a completely satisfactory stability of viscosity or cement stability, and which do not hinder the setting of cement. Corresponding cementitious construction compounds ought, therefore, to have outstanding processing properties and to lead to buildings having very good mechanical properties, such as tensile adhesive strengths.